Axial temperature gradients in phosphoric acid solutions under hydrothermal conditions have been measured, which occur due to different directions of the heat flux gradient forced upon the system. The results were analysed and interpretated by means of numerical modelling the heat transfer and the flow field. Additionally, forced convection was considered by calculating the influence of a rotating solid body (crystal) inside the calculation domain. The results are discussed with respect to their consequences on crystal growth of AlPO4 and GaPO4 if the temperature gradient method is to be utilized. (C) 2002 Elsevier Science B.V. All rights reserved.